1. Field of the Invention
The present invention generally relates to the field of couplers which are intended to extract data proportional to a signal carried by a transmission line.
The present invention more specifically relates to couplers formed by means of conductive lines coupled to each other with no contact. Such couplers are called couplers with distributed lines as opposed to couplers with local elements, formed from capacitive and inductive elements.
The present invention more specifically applies to the field of radio-frequency couplers, for example, for radio-communication applications of mobile telephony type.
2. Discussion of the Related Art
FIG. 1 shows a conventional example of a coupler 10 with distributed lines. A main line 11 connects an input access IN to an output access DIR. Line 11 forms the primary of the coupler and is intended to convey the useful signal. A secondary line 12 is arranged parallel to line 11 to ensure a contactless coupling therewith to sample part of the power present on line 11. The two ends of line 12 define accesses, respectively CPLD intended to interpret the result of the coupling and ISO, generally isolated, that is, in the air. The coupler is typically formed by metal tracks deposited on an insulating substrate.
A distributed coupler is generally characterized by the following parameters:
the transmission losses between terminals IN and DIR;
the coupling, which corresponds to the transmission losses between terminals IN and CPLD;
the isolation of the coupling which corresponds to the transmission loss between terminals DIR and ISO; and
the directionality, which represents the difference in decibels between the signals present on terminals ISO and CPLD.
The first three above parameters are generally measured while the two terminals not taken into account are loaded with standardized impedances (generally 50 ohms).
The lengths given to the main and secondary lines are calculated according to the central frequency of the passband for which the coupler is intended and to the desired coupling. Typically, these lines have lengths corresponding to one quarter of the wavelength of this central frequency. The longer the lines, the greater the insertion losses.
FIG. 2 very schematically shows, in the form of blocks, a radiofrequency transmission chain of the type to which the present invention applies as an example. A transmit amplifier 1 (PA) receives a radiofrequency signal RF to be transmitted by an antenna 2. To control the transmit power to a value, set by a reference REF, a coupler 10 with distributed lines between the output of amplifier 1 and antenna 2 is used. Accesses IN and DIR of main transmission line 11 are respectively connected to the output of amplifier 1 and to the input of antenna 2. Terminal CPLD of the coupled line is connected to the input of a detector 2 (DET) having its output compared (comparator 4) with reference signal REF to adjust the transmit power (the gain) of amplifier 1.
In a so-called directional coupler, a signal entering through terminal DIR is trapped by terminal ISO to avoid that this signal reaches the application, for example, amplifier 1 (FIG. 2). In this case, terminal ISO is generally loaded with a grounded 50-ohm impedance. “Higher directionality” is used to designate a greater attenuation in dB between accesses ISO and CPLD.
In other cases, an external isolator is provided between coupler 10 and antenna 2 to prevent a return of the signal to amplifier 1. The coupler then needs not be directional and terminal ISO is generally left in the air.
The present invention more specifically relates to directional couplers.
A disadvantage of couplers of the type illustrated in FIG. 1 is that the coupling is very sensitive to the frequency of the transmitted signal.
This disadvantage is particularly disturbing in radiocommunication applications more specifically aimed at by the present invention. Indeed, too high a variation of the coupling within the same operating frequency band (for example, GSM or DCS) adversely affects the optimization of the transmission chain operation. Further, the coupling may vary significantly from one frequency band to another.
A directional coupler is described, for example, in patent application No. US-A 2004/0113716 of the applicant. This coupler has interdigited transmission lines, and is also known as a Lange coupler. As compared with couplers with non-interdigited lines, a Lange structure enables improving the coupling between lines.
“Improvement in the coupling” is used to mean an increase in the attenuation in dB of the signal on terminal CPLD with respect to the desired signal to draw as little as possible from this signal.
“Improvement in the directionality” is used to mean an increase in the attenuation in dB of the signal on terminal ISO with respect to terminal CPLD.
Traditionally, to improve the directionality, capacitive elements are provided either between terminals of the coupler, or between some of these terminals and the ground.
A disadvantage is that, in frequency bands aimed at by the present invention, the values of the capacitive elements are so low that they become close to the values of the stray capacitances of the structure, which makes the coupler difficult to form.